Magnetic flow meter circuit utilizing field effect transistors



April 12, 1966 A. NAZARETH, JR

MAGNETIC FLOW METER CIRCUIT UTILIZING FIELD EFFECT TRANSISTORS OriginalFiled Oct. 19. 1962 XEOZEMZ On- AI wk mm Efrem-m nwmzww 6 5.. :55.aumzuw e mmimomwzaih INVENTOR Alfred JVZzzare/gfi:

BY m mm w '54 J AT NEYS United States Patent MAGNETHC FLOW METER CIRCUITUTILIZTNG FIELD EFFECT TRANSESTDRS Alfred Nazareth, J22, Rehohoth, Mass,assignor to The Foxhoro Company, Foxhoro, Mass. Continuation ofapplication Ser. No. 231,711, Get. 19, 1962. This application Mar. 8,1965, Ser. No. 443,752 Claims. (Cl. 30788.5)

This application is a continuation of my application, Serial No.231,711, filed October 19, 1962.

This invention relates to signal translating apparatus. Moreparticularly, this invention relates to apparatus for producing atransmittable output signal responsive to the amplitudes of two signals.In the disclosed embodiment, the invention is applied to a magnetic flowmeter transmitter and operates to maintain the transmitted output signalproportional to flow rate in spite of variation in the A.-C. voltagesupplied to the magnetic field coils.

In the A.-C. magnetic flow meter the true measure of fiow velocity isthe ratio of the magnitude of the A.-C. fiow signal to the magnitude ofthe A.-C. voltage supplied to the field coils. This is because the howsignal is proportional to the product of fiow velocity and magneticfield and the latter varies directly with voltage applied to the fieldcoils. Thus, if an instrument is used which measures simply themagnitude of the flow signal voltage, it will be necessary to supply thefield coils from an accurately regulated A.-C. supply, an elaborate andexpensive device particularly in the case of a magnetic flow meter withits large power requirements. On the other hand, using an instrumentwhich measures the ratio of flow signal to field coil supply voltagerenders unnecessary the use of an A.-C. regulator and allows theconstruction of a much simpler and more economical fiow meter system.The system considered here will convert the millivolt per volt signalfrom an A.-C. magnetic flow meter, or any ratio of two A.-C. voltages,to a transmitta-ble D.-C. output. It could be used with a resistancethermometer bridge with an A.-C. supply.

In such a system an A.-C. error signal, the difference between the A.-C.fiow signal and an A.-C. feedback signal obtained from the output, isfed to the attenuation network through a high-gain amplifier and throughphase shifters, rectifiers, and such other devices as are required toobtain a suitable control signal for the attenuation network. The D.-C.output from the attenuation network is then taken as the requiredoutput; the A.-C. output is taken as the feedback signal. Now theattenuation network has the property that:

D.-C. Output D.-C. Supply A.-C. Output, A.-C. Supply The high-gainamplifier operating in the feedback loop will tend to minimize the errorsignal, and therefore, to maintain an essential equality between theA.-C. output of the attenuation network, the feedback signal, and theA.-C. flow signal. Using this equality, and rearranging the previousequation, there results:

A.- C. Flow Signal A.-C. Supply D.C. Output: D.-C. Supply In suchapplication it is, of course, important that an exact proportionalityexist, in the attenuation network, between the degree of attenuation ofthe A.-C. supply and that of the D.-C. supply, since it is on thisrelationship that the accuracy of the final measurement depends.Notwithstanding such performance requirements, this device should alsohe relatively simple, reliable in operation, and capable of beingproduced economically.

Accordingly it is an object of this invention to provide improved signaltranslating apparatus. The following description should be consideredtogether with the accompanying drawing, which is a diagrammaticrepresentation of a flow meter system incorporating a preferredembodiment of the invention. While this description deals specificallywith the application of the invention .to a flow meter system, otherspecific objects, aspects, and advantages of the invention will beapparent from, or will be pointed out in the description which follows.

Referring now to the drawing, there is shown a conventional magneticflow head 10 comprising a flow pipe 12 having electrodes 14 on oppositesides thereof, and a pair of coils 16 connected to the A.-C. power lineto produce a magnetic field within the pipe. The electrodes 14 areconnected through a pair of leads 18 to a balanced differencetransformer 20, e.g., as shown in detail in US. Patent 3,006,188. Thistransformer combines the A.-C. fiow signal on leads 18 and the negativefeedback signal from leadsZl, and produces on leads 22 a signalcorresponding to the difference between the flow signal and the feedbacksignal. 7

This difference signal on leads 22 is fed to the input of an amplifier24, for example a transistor amplifier as shown in the above-identifiedCushman patent. This amplifier also includes a conventional phasingnetwork (not shown herein) which corrects for phase shift introduced bythe magnetic fiow head 10, and produces at the amplifier output an A.-C.signal which is in phase (or 180 out-of-phase) with respect to the A.-C.line voltage.

This amplified A.-C. signal is fed to a transformer 26 having a pair ofisolated secondaries 28 and 30. These secondaries are connected torespective phase-sensed rectifiers 32 and 34 which are supplied withcomparison signals from the A.-C. power line. The two rectifier circuitsare so arranged that they produce a zero D.-C. output when the A.-C.input signal fed thereto is zero or out-of-phase with respect to theA.-C. line voltage, and therefore with respect to the amplified tlowsignal. Each rectifier produces a D.-C. output of one polarity when theinput signal is in phase with the line voltage and a D.-C. output ofopposite polarity when the in ut signal is out-of-phase with the linevoltage. The use of phasesensed rectifiers not only renders the systemsensitive to the phase of the flow signal, and therefore to thedirection of the flow, but renders it insensitive to quadrature signals,spurious signals 90 outof-phase with respect to the flow signal, thebulk of which are generated by magnetic induction in the flow meterhead. The output circuits of these rectifiers are oppositely polarized,that is to say, for a given phase of input signal, say in phase with theline voltage, the D.-C. output of the upper rectifier 32 might vary fromzero to plus 4 volts, while the output of the lower rectifier 34 variesfrom zero to minus 4 volts.

The D.-C. output potentials of the phase-sensed rectifiers 32 and 34 arefed in as control signals to an attenuation network consisting in thiscase of the two fieldefiect transistors 41 and 43 and their associatedcomponents. The field effect transistors have gate electrodes 49 and 42,drain electrodes 44 and 46, and source electrodes 48 and 50.Field-effect transistors are well known in the art (see, for example,US. Patent 3,001,111, 3,007,119 and 3,010,033), and the characteristicstheresesame of are available in various publications. One of theimportant characteristics of field-effect transistors is that therelationship between drain-source voltage and drain current at lowdrain-source voltage, and at any particular value of gate-sourcevoltage, is essentially that of a pure resistance, at least over alimited range of operation. That is to say, the A.-C. incrementalresistance, the slope of the drain characteristic at a particular point,is equal to the D.-C. static resistance, the ratio of voltage to currentat that point. In addition, this efiective resistance varies markedlywith changes in the voltage applied to the gate electrode, e.g., thisresistance may change by a factor of 100011 for a moderate variationsuch as 4 volts, in the potential applied to the gate electrode. Theseare the characteristics required for proper operation of the attenuationnetwork. Other devices with the same pure resistance characteristic suchas photo-diodes, thermistors, and magneto-resistive devices might wellbe equally suitable.

The two field-effect transistors 41 and 43 are connected together in aform of bridge circuit. The common junction 52 therebetween is connectedthrough a load resistance 54 to the movable arm of a potentiometer 58.This potentiometer is connected across a D.-C. energizing source 60, andbetween a pair of secondary windings 62 and '64 of a transformer 66, theprimary of which is connected to the A.-C. power line. The remote endsof these are connected respectively to the drain electrode 44 of theupper field-effect transiston In essence this attenuation networkfunctions as a bridge, the two fieldeffect transistors, or alternatedevices, forming a voltage divider fed simultaneously from an AC. and aD.-C. supply. If this network is to have the required characteristics,that is, that the ratio of A.-C. output to A.-C. supply be equal, or atthe very least proportional, to the ratio of D.-C. output to D.-C.supply, then it is important for a practical device that the voltagedivider elements, the field-efiect transistors or alternate devices,function as variable pure resistors. That is, the effective drain-sourceresistance, in the case of field-effect transist-ors, must be the samefor both A.-C. and D.-C. at any given control signal. If this conditionis satisfied then the ratio A.-C. output to A.-C. supply is, in fact,equal to the ratio D.-C. output to D.-C. supply.

When there is no A.-C. signal at the output of the amplifier 24, i.e.,when there is no flow through the pipe 12, the potential applied to thegate electrodes 40 and 42 will be equal, e.g., minus 4 volts. Thus theresistances presented by the field-effect transistors 41 and 43 will beequal, and accordingly the potential of the common junction 52 will beessentially midway between the potentials of the drain electrode 44 andthe source electrode 50. The potentiometer 58 is adjusted so that, inthis condition, the potential of its movable arm 56 is equal to thepotential of junction 52. Thus, there will is): no A.-C. or D.-C. outputacross the load resistance When liquid fiows through the pipe 12, theresulting A. C. output of the amplifier 24 will produce correspondingD.-C. potentials at the outputs of the phase-sensed rectifiers 32 and34, e.g., plus 1 volt from rectifier 32 and minus 1 volt from rectifier34. The potentials applied to gate electrodes 44) and 42 will changecorrespondingly, e.g., to minus 3 volts at electrode 40 and minus 'voltsat electrode 42. Thus, the resistance of field-effect transistor 41 willdecrease and the resistance of fieldettect transistor 43 will increase,with the result that the potential of junction 52 will no longer beequal to the gtential of the movable arm 58 of the potentiometerAccordingly, an output signal will appear across load resistance 54 witha magnitude corresponding to the DC. potential produced by thephase-sensed rectifiers 32 and 34. This output signal will comprise anA.-C. output signal and a D.-C. output signal, since the fieldeffecttransistors are supplied with current from both an A.-C. energizingcircuit (transformer 66) and a D.-C. energizing circuit (D.-C. supply60). If the impedance of the sensing circuit changes, as with a changein the resistances of the field-effect transistors, and provided boththe A.-C. and D.-C. supplies remain constant, the resulting change inthe D.-C. output signal will be accompanied by an exactly proportionalchange in the A.-C. output signal, since both output signals aredeveloped by the flow of current through the same circuit elements.

The A.-C. signal developed across the load resistance 54 is fed througha phase network 70 and leads 21 to the difference transformer 2G tooppose the A.-C. flow signal received from leads 18. The phase network70 adjusts the phase of this A.-C. feedback signal so that it matchesthe phase of the A.-C. flow signal. Consequently, there will be a nullat the input to amplifier 24, since this amplifier has a very high gain,and the A.-C. feedback signal will have effectively the same amplitudeas the A.-C. flow signal. With this relationship es tablished, anychange in the A.-C. fiow signal resulting from a change in flow ratewill necessarily result in an exactly corresponding change in the D.-C.output signal across load resistance 54, since this D.-C. output signalwill track the A.-C. output signal across the load resistance. However,if the A.-C. flow signal changes as a result of variations in the AC.line voltage supplying the coils 16 there will be substantially noetiect on the D.-C. output signal because the line voltage variationwill produce a corresponding change in the A.-C. feedback signal, thustending to prevent any variation in the difference signal on leads 22.In other words the device is insensitive to line voltage variationsbecause it measures the ratio of how signal to line voltage.

The D.-C. output signal across load resistance 54 is fed through afilter network, consisting of resistor 72 and capacitor '74, to a pairof output terminals 76 and 78. From there, the D.-C. signal may betransmitted to a remote station, possibly with additional amplificationif required, and applied to a flow indicating or recording in strumentin a conventional manner.

It should be understood that the disclosure herein is only intended asan illustration of the present invention, and it is apparent thatnumerous modifications may be made Within the scope of this invention.For example, the magnetic flow meter transmitter may include a biasarrangement in order to provide the D.-C. output signal with a livezero. Also, it should be understood that the showing herein isdiagrammatic in certain respects, e.g., the D.-C. sources 36, 38 and 6%are indicated as batteries, but in an actual transmitter such D.-C.sources will consist of conventional electronic D.-C. power supplies thedetails of which are well known.

I claim:

1. Electrical signal-responsive apparatus adapted for use as a D.-C.transmitter and comprising, in combination, an amplifier forintensifying an A.-C. input signal, rectifier means connected to theoutput of said amplifier for developing a D.-C. potential responsive tochanges in the amplitude of said A.-C. input signals; two field-effecttransistors each having a pair of current-carrying electrodes and a gateelectrode, first circuit means connecting a currentcarrying electrode ofone of said field-effect transistors to a current-carrying electrode ofthe other transistor; A.-C. and D.-C. energizing means having a pair ofprincipal output terminals providing both A.-C. and D.-C. energizingvoltages and a third output terminal providing a potential intermediatethe potentials of said principal output terminals; means connecting saidpair of principal output terminals respectively to the othercurrent-carrying electrodes of said field-effect transistors to producea flow of Arc. and D.-C. current through said two transistors in series;second circuit means for providing to said gate electrodes controlsignals corresponding to said D.-C. po-' tential, said second circuitmeans including means to pro vide an inverse relationship between saidcontrol signals such that as the resistance of one of said field-eifecttransisters is increased the resistance of the other of said transistorsis decreased; an impedance element coupled between said first circuitmeans and said third output terminal of said means to produce acrosssaid impedance element A.-C. and D.-C. output signals corresponding tothe potential of said first circuit means as determined by the relativeresistances of said two field-efiect transistors; and negative feedbackmeans coupled to said impedance element and arranged to direct to theinput of said amplifier a feedback signal corresponding to said A.-C.output signal.

2. Apparatus as claimed in claim 1, wherein said A.-C. energizing meanscomprises a transformer having a twosection secondary winding, theremote ends of said secondary winding sections being coupled to saidother current-carrying electrodes and the adjacent winding ends beingcoupled to one terminal of said impedance element, the other terminal ofsaid impedance element being connected to said first circuit means.

3. Apparatus as claimed in" claim 2 wherein said D.-C. energizing meanscomprises a D.-C. power supply connected between said winding sections.

4. Apparatus as claimed in claim 3 including a potentiorneter connectedacross said DC. power supply, the movable arm of said potentiometerbeing connected to said one terminal of said impedance element.

5. Apparatus as claimed in claim 1 wherein said rectifier meanscomprises a pair of phase-sensed rectifiers each having an outputconnected to the gate electrode of a respective field-effect transistor.

6. For use in an industrial process instrumentation sys tem, anelectronic transmitter for producing a direct-cup rent output signalcorresponding to the value of a process condition and adapted for usewith condition-sensing means of the type energized by an A.-C. supplysource to produce an AC. measurement signal responsive to the value ofthe process condition and also to the amplitude of the A.-C. supplyvoltage; said transmitter comprising, in combination, a high gainamplifier to receive said A.-C. measurement signal, a variableattenuation network including means responsive to the amplifier outputsignal for altering the attenuation of said network in accordance withchanges in the AC. input signal fed to said amplifier; a D.-C. powersupply; circuit means coupling said A.-C. supply source and said D.-C.power supply to the input of said attenuation network to producecorresponding A.-C. and D.-C. output signals at the output of saidattenuation network, whereby the amplitude of the A.-C. output signal ismade responsive both to the output of said amplifier and to theamplitude of the A.-C. supply voltage and the magnitude of the D.-C.output signal correspondingly is made responsive both to the output ofsaid amplifier and the magnitude of the D.-C. supply voltage; andnegative feedback means coupling said A.-C. output signal to the inputof said amplifier in opposition to said A.-C. measurement signal.

7. Signal-responsive apparatus comprising an amplifier adapted toreceive an input signal; a variable attenuation network having an inputand an output and including at least one variable impedance element theimpedance of which controls the attenuation presented by said network,said variable impedance element having a control terminal independent ofsaid input and output of said network; first circuit means for directingto said control terminal a signal responsive to the output of saidamplifier for controlling the impedance of said variable impedanceelement and thereby controlling the amount of attenuation presented bysaid network; AC. and D.-C. energizing means coupled to the input ofsaid attenuation network to produce at the output of said networkcorresponding A.-C. and D.-C. ouput signals the magnitudes of which varytogether with changes in the amount of attenuation presented by saidnetwork; and negative feedback means coupling one of said output signalsto the input of said amplifier.

8. Apparatus as claimed in claim 7, wherein said impedance element is anelectronic device the resistance of which varies with changes in anapplied control signal.

9. For use in an industrial process instrumentation system, anelectronic transmitter for producing a directcurrent output signalcorresponding to the value of a process condition and adapted for usewith condition-sensing means of the type energized by an A.-C. supplysource to produce an A.-C. measurement signal responsive to the value ofthe process condition and also to the amplitude of the A.-C. supplyvoltage; said transmitter comprising in combination: a high-gainamplifier arranged to receive said A.-C. measurement signal and toproduce a corresponding output signal; a variable attenuation networkhaving an input circuit to receive electrical signals to be attenuated,impedance means forming part of said network and connected in the pathof current flow produced by said electrical signals, an output circuitcoupled to said impedance means to produce an output voltage from saidnetwork responsive to the amount of said current flow; said networkimpedance means including a controllable impedance element with meansresponsive to said amplifier output signal for setting the impedance ofsaid element in correspondence to the magnitude of said amplifier outputsignal, thereby causin the attenuation of said network to be altered inaccordance with changes in the A.-C. input signal fed to said amplifier;a D.-C. power supply; circuit means coupling said A.-C. supply sourceand said D.-C. power supply to said input circuit of said attenuationnetwork to produce corresponding A.-C. and D.-C. output voltages at saidnetwork output circuit, whereby the amplitude of said A.-C. outputvoltage is proportional both to the output signal of said amplifier andto the amplitude of the A.-C. supply voltage and the magnitude of saidD.-C. output voltage correspondingly is proportional both to the outputsignal of said amplifier and the magnitude of the D.-C. supply voltage;and negative feedback means for developing an A.-C. feedback signalcorresponding to said A.-C. output voltage and for coupling saidfeedback signal to the input of said amplifier in opposition to said A.C. measurement signal.

10. Signal-responsive apparatus comprising amplifier means adapted toreceive an input signal; a variable attenuat-ion network having an inputcircuit to receive electrical signals and including impedance meansconnected in the path of current flow developed by said electricalsignals, an output circuit for said network and coupled to saidimpedance means to develop output signals corresponding to saidelectrical signals; said impedance means including a resistive elementthe resistance of which is controlla-bly variable; means responsive tothe output of said amplifier for controlling the resistance of saidelement in correspondence with said amplifier output, thereby to controlcorrespondingly the attenuation produced by said network; A.-C. andD.-C. energizing means coupled to said input circuit of said attenuationnetwork to supply thereto corresponding electrical energizing signalshaving alternating and non-alternating directional characteristics,respectively, said energizing signals producing current fiow throughsaid impedance means and developing at said output circuit of saidnetwork corresponding A.-C. and D.-C. output signals, the amount of saidcurrent flow being variable with changes in the resistance of saidcontrollable element as determined by said amplifier output so that saidA.-C. and D.-C. output signals vary in correspondence to changes in saidinput signal applied to said amplifier means; and negative feedbackmeans for said amplifier means, said feedback means coupling one of saidamplifier output signals in opposition to said input signal, said oneoutput signal having the same directional characteristics as saidamplifier input signal so that the resultant signal applied to the inputof said amplifier represents the difference in magnitude between saidinput signal and the feedback signal developed by said feedback means.

11. Apparatus as claimed in claim 10, wherein said impedance elementcomprises a field-effect transistor having a gate electrode which iscoupled to the output of said amplifier, so that the resistance of saidtransistor is controllableby said amplifier output.

12. Apparatus for use with A.-C. condition-measurement systems, such asa magnetic flowmeter, to convert an A.-C. condition-responsive signal toa corresponding D.-C. output signal, said apparatus comprising, incombination, an amplifier for intensifying the A.-C. signal, rectifiermeans connected to the output of said amplifier for developing a D.-C.potential responsive to changes in the amplitude of said A.-C. signal;an attenuation network including a field-effect transistor and animpedance element connected in series therewith, circuit means forfeeding said D.-C. potential to the gate electrode of said field-effecttransistor, A.-C. and D.-C. energizing means coupled to said attenuationnetwork to produce corresponding A.-C. and D.-C. current iiow throughsaid fieldefiect transistor and said impedance element in series, theresulting A.-C. and D.-C. output signals across said impedance elementhaving magnitudes controlled by the resistance of said field-effecttransistor as determined by the D.-C. potential applied to said gateelectrode; and negative feedback means coupling said A.-C. output signalto the input of said amplifier.

13. Apparatus as claimed in claim 12, wherein said impedance elementcomprises a resistor.

14. For use in an industrial process instrumentation system, anelectronic transmitter for producing a direct current output signalcorresponding to the value of a process condition, said transmitterbeing adapted for use with condition sensing means of the type havingelectrical circuit means energized by an A.-C. supply source forproducing an A.-C. measurement signal which is responsive to the valueof the process condition and also to the amplitude of the A.-C. supplyvoltage, the meas urement signal being fed to a high gain amplifier theoutput circuit of which includes means to produce the di- E5 rectcurrent output signal, said output circuit also including means toproduce an AC. feedback signal derived from said A.-C. supply andcombined with said A.-C. measurement signal to develop for saidamplifier a resultant A.-C. input signal which represents the differencebetween said A.-C. feedback signal and the A.-C. measurement signal;said amplifier output circuit means comprising an all electronicvariable attenuation network for simultaneous attenuation of twovoltages derived respectively from said A.-C. supply and from a D.-C.supply, so as to produce said A.-C. feedback signal and said directcurrent output signal with magnitudes in the same ratio as that of theA.-C. and D.-C. supply voltages, and fully electronic means incorporatedin the variable attenuation network and responsive to the output of saidamplifier to alter the attenuation of said network in accordance withchanges in the A.-C. input signal to said amplifier, the relationbetween said A.-C. supply voltage and said A.-C.

feedback signal thus being caused to closely approximate the relationbetween the A.-C. supply voltage and the A.-C. measurement signal of thesensing means, thereby maintaining the relation between the D.-C. outputsignal and the D.-C. supply of the variable attenuation network at avalue representing the value of'the measured condition of the process.

15. Apparatus as claimed in claim 14 wherein said variable attenuationnetwork includes at least one purely resistive element connected to aload element, both of said elements being energized by said A.-C. andD.-C. supplies to produce across said load element A.-C. and D.-C.signals the magnitudes of which vary correspondingly with changes in theresistance of said purely resistive element, and from which said A.-C.feedback signal and said direct current output signal are derivedrespectively.

References Cited by the Examiner UNITED STATES PATENTS 2,511,855 6/1950Keck et al 330-114 3,006,188 10/1961 Handel et al. 73194 3,131,5605/1964 Cushman et al. 73-194 ARTHUR GAUSS, Primary Examiner.

R. H. EPSTEIN, Assistant Examiner.

7. SIGNAL-RESPONSIVE APPARATUS COMPRISING AN AMPLIFIER ADAPTED TORECEIVE AN INPUT SIGNAL; A VARIABLE ATTENUATION NETWORK HAVING AN INPUTAND AN OUTPUT AND INCLUDING AT LEAST ONE VARIABLE IMPEDANCE ELEMENT THEIMPEDANCE OF WHICH CONTROLS THE ATTENUATION PRESENTED BY SAID NETWORK,SAID VARIABLE IMPEDANCE ELEMENT HAVING A CONTROL TERMINAL INDEPENDENT OFSAID INPUT AND OUTPUT OF SAID NETWORK; FIRST CIRCUIT MEANS FOR DIRECTINGTO SAID CONTROL TERMINAL A SIGNAL RESPONSIVE TO THE OUTPUT OF SAIDAMPLIFIER FOR CONTROLLING THE IMPEDANCE OF SAID VARIABLE IMPEDANCEELEMENT AND THEREBY CONTROLLING THE AMOUNT OF ATTENUATION PRESENTED BYSAID NETWORK; A.-C. AND D.-C. ENERGIZING MEANS COUPLED TO THE INPUT OFSAID ATTENUATION NETWORK TO PRODUCE AT THE OUTPUT OF SAID NETWORKCORRESPONDING